Triple-negative breast cancer (TNBC) accounts for approximately 15% of invasive breast cancers and disproportionately affects African-American and younger women. These patients have high risk of recurrence and metastasis. Treatment options remain limited and new targeted agents are urgently needed. We and others have shown that y-secretase inhibitors (GSI) are active in breast cancer preclinical models, including TNBC. GSIs are being tested in TNBC based on the notion that Notch and other y-secretase substrates play important roles in breast cancer biology. However, critical knowledge gaps may compromise the clinical development of GSIs in TNBC. This project will test the hypothesis that: A) GSIs affect tumor growth both directly and through tumor host effects that vary between the drugs in this class; and B) the effects of GSIs are at least in part mediated through cross-talk with the VEGF and AKT pathways. Our data show that GSIs differ in the quality and potency of their effects on TNBC cells, endothelial cells and T-cells. There was no obvious correlation between the ability of GSIs to inhibit expression of canonical Notch target HES1 and their biological activity in vitro or in patients. GSIs have significant pharmacologic interactions with tyrosine kinase inhibitor sunitinib and AKT inhibitor perifosine. A GSI-perifosine combination is more active in TNBC xenografts than either drug alone. GSI inhibit Th17 T-helper responses, which play a crucial role in tumor development and angiogenesis. We plan to compare several investigational GSIs including Notch-sparing and Notch-selective agents to each other and to direct Notch inhibitors such as dominant negative MAML1 peptides in TNBC cells, endothelial cells and T-cells. We will explore the effects of selected GSIs, alone and in combination with sunitinib or perifosine, on angiogenesis and T-helper responses in tumors, using a novel transgenic mouse model of TNBC. We will identify candidate biomarkers of anti-tumor activity to be tested in the clinic. Finally, we will explore the role of Notch-mediated AKT activation in the anti-tumor activity of GSIs in TNBC.